1. Field of the Invention
This invention relates, in general, to printing and reproduction apparatus and, more specifically, to image frame length control for images produced in hard copy scanning printers, including laser printers, and other apparatus having photosensitive media.
2. Description of the Prior Art
In high speed electrostatographic reproduction apparatus, it is a common practice to employ photosensitive media in the form of an elongated photoconductive web (or a photoconductive coating on a drum) adapted to record transferable images. Such media moves in a path in operative relation with various electrophotographic process stations. Ultimately a transferable image is created and transferred to a receiver to produce a print or reproduction.
In making monochrome reproductions with an apparatus utilizing a uniformly charged photosensitive media, areas of uniform charge are exposed to light to form a charge pattern, or latent image frame. In the color reproduction arts, the image frame can be designed to correspond to one of a set of related color separation images; several successive image frames on the media may thus constitute a separations set. The latent images are developed with pigmented marking particles to form transferable images. Each image is transferred sequentially to a respective receiver member that may in turn be used as one of the several color separations for a composite multicolor reproduction. Alternatively, each image may be transferred directly to a single receiver to create a multichromatic (multicolor) print in one step.
The sequential separation images must be accurately registerable in order to obtain faithful multicolor reproductions. In such color applications, the transferable images generated from such successive "master" separations must be aligned for accurate superimposed registration during the creation of a multicolor composite print.
Even monochrome reproductions that are not intended for use in composite images may require accurate reproduction of finely-spaced lines or other minute, repetitive image components. Such components in a detailed original (for example, a map or chart) must be reproduced with fidelity to the original image. Non-uniformity in the writing of the latent image on the media can cause a noticeable image degradation: a finely ruled reticular grid in a reproduction of an image will appear uneven in thickness and spacing; a reproduction of an image having a grey-scale pattern will have noticeable density gradient variation as well.
In a web-based reproduction or printing apparatus, the web is typically supported by several freely turning rollers and driven by one drive roller. (In alternative reproduction apparatus, a driven drum assembly is substituted for the web and rollers.) Because these driving assemblies are electro-mechanical systems, there is a tendency for the web (or drum) to vary in speed as it is driven. Moreover, because the web or drum is photosensitive and typically is exposed line-by-line by a laser beam or a linear LED array, any speed variation of the media will cause the exposure lines to be written at differing inter-line spacing. The sum of this effect is that different image frames on the photosensitive media will have images of differing length. Those image frames, when developed and transferred, will produce reproductions with correspondingly disparate frame lengths.
Referring now to FIG. 1 in particular, there is shown a reproduction apparatus 8 which includes a laser scanner 10 for emitting a laser beam 12. An LED linear array and other non-laser light sources are also useable. A photosensitive media, in this example a photoconductive web 14, is constructed in an endless loop and disposed around rollers, such as rollers 16, and is rotated in the direction indicated by arrow 18. The exposing or imaging station 20 includes a driving means, such as a drive roller 22. For clarity, the drive roller 22 is assumed to include all other required motor apparatus not specifically illustrated in FIG. 1. During the operation of the printer, the web 14 travels across roller 22 and is cleaned by a brush 18 and charged at a charging station 19. The web is then exposed, line by line, by the laser beam lZ which is controlled by suitable electronics to construct a desired electrostatic latent image Frame on the web 14. This latent image frame moves, with the movement of the web 14, through other stations of the printer. At the development station 24, toner is applied to the web 14, and at the transfer station 26, the developed image on the web 14 is transferred, with the aid of the transfer roller 28, to a receiver. The receiver travels along the guide 32 and between the rollers 16 and 28 for transfer of the image from the web 14. Further details on the illustrated printer may be found in the aforementioned U.S. application Ser. No. 248,075.
As further example, it will be noted that a drum-based system may be found to operate similarly, in that the web 14 and rollers 22 and 16 may be replaced by a photosensitive drum of sufficient circumference to engage the electrophotographic process stations in the same manner. The drum is also driven by a motor and exposed to the laser beam 12 to receive a latent image at its surface on a line-by-line sequence. For clarity, however, the background of the present invention will be discussed with reference to the illustrated web-based system.
In the production of a latent image frame, any speed variation of the photosensitive media must be held to a minimum; otherwise, the successive latent image frames are likely to have differing lengths. The resulting misregistration of the developed and transferred image frames must be held within acceptable limits. Gross variations in web speed are caused by a variety of mechanical factors which affect the web transport speed, such as roller bearing or drive motor friction, line voltage changes, assymetry of the drive motor poles, or misalignment of the apparatus support chassis with respect to the axes of the web transport rollers and the web drive means. Much improvement has been made in the art to reduce such gross speed variation to a level that is acceptable for most printing and reproduction applications.
In Mager et al., U.S. Pat. No. 4,835,545, a photosensitive media moving in a first direction, relative to a laser light beam scanning in a second direction, incurs velocity variation which cause variations in the absolute and relative heights of white and black image features. An instantaneous velocity error calculation is used to adjust the intensity of the laser light beam to be proportionally brighter (dimmer), exposing a wider (narrower) scan line, on a faster-moving (slower-moving) media region.
In Hoshino, et al., U.S. Pat. No. 4,803,515,an image forming apparatus includes a movable image bearing members and a driver for driving the image bearing member. The time interval required for the image bearing member to move from a latent image forming position is an integer multiple of a period of the drive non-uniformity inherent in the driver.
In Lama et al., U.S. Pat. No. 4,801,978, a control circuit is provided in an electronic printer utilizing an image write bar to compensate for the effects of vibration in a rotating photoconductive member. In Kramer et al., U.S. Pat. No. 4,785,325, a document imaging system incorporates a mechanism for adjusting the speed ratio between the document scanning system and the photoreceptor. In Ritter et al., U.S. Pat. No. 4,779,944, an integrated laser scanning system for scanning a modulated beam across an image surface (receptor) is provided. The variation in rotational speed of the receptor is discussed as a major source of scan line spacing error in images recorded with a laser scanner.
In Hanlan, U.S. Pat. No. 4,361,260, a register control is provided for a web handling apparatus wherein a control provides a timed modification to a sensed speed signal for providing a modified speed command signal when the system is out of registration. In Draugelis et al., U.S. Pat. No. 4,082,443,digital logic circuitry ensures that latent images are correctly placed on a photoconductor, by varying the timing of the flash assembly.
In Kushner, U.S. Pat. No. 3,934,505, a method and apparatus are disclosed wherein a signal proportional to the speed of a moving web is compared with a signal proportional to the speed of a motor-driven rotary printing member, and a resultant corrective signal is transmitted to the control for a motor. The linear speed of the rotary printing member is thus made equal to the web speed.
A residual amount of media transport speed variation has, until now, been assumed to be irreducible. This residual speed variation has generally been unrecognized or unimportant in the many prior art printing or copying machines which produce single images that are not of critical fidelity. However, all image frames that are produced on a machine having this speed variation will have varying frame lengths. The length variation becomes most obvious and objectionable when the image frames are used to reproduce a color document. One image may be developed and transferred to a receiver for use as one in a set of color separations or color masters that are employed in a second machine, such as a xeroprinter, to make a color print. Or, several images may be developed and transferred in superposition onto a single receiver to make a composite color print. In both instances, the disparity of the image frame lengths will cause a noticeable misregistration and color shift in the color print.
While perfecting the web transport of the illustrated reproduction apparatus 8, we have found much, if not all, of this residual speed variation to be due to a variation of the surface velocity of the photosensitive media. More importantly, we have found this variation in web surface velocity to be directly attributable to variations in the thickness of the web.
As shown in FIGS. 1a and 1b, a driving means, such as the drive roller 22 in the web-based system, or the drum core 23 in a drum-based system, may be assumed to rotate at a constant angular velocity .mu.. The web transport speed v is observed at points on the midpoint depth d of the material. A radius, taken from the arc described by the path d to the center axis of a drive means, will increase and then subside to normal as a relatively thicker section passes around the drive means. Simultaneously, because the web is continuous, the surface velocity of all points :n the media also increases and then subsides. The same radius will decrease when a thin section passes over the drive means and respectively the web transport speed (and the surface speed) will decrease. Therefore, an image written to any area on the web during such surface speed changes will have a frame length that is respectively increased or decreased.
As noted above, this phenomena is due to the nature of the transport speed of a continuous material driven in an endless configuration. With reference again to FIGS. 1A and 1B, any surface point on such a continuous media (the web 14 or the drum coating 23a) will have an increase in surface velocity V.sub.2, as the thick part of the web passes over the driving means. Conversely, the media will have a lower surface velocity (V.sub.1) when a thin section of the web passes over the drive means. When a section of the web having a median thickness (neither thick nor thin) passes over the drive roller, the surface speed V.sub.s is at a median speed which is an amount between V.sub.1 and V.sub.2. (In a drum-based system, the thickness variation is definable as the variation of the radius of the drum 23 which includes photosensitive coating 23a; the drum 23 cross-section may thus be understood as non-circular. This variation is illustrated by radius r.sub.1 or r.sub.2 drawn from the center of rotation c to respective surface points.)
Although a subtle phenomena, the residual speed variation is repeatable and predictable. In researching the speed variation on a 3-image frame web in the illustrated apparatus 8, 306 prints were generated and then measured for image length with a precision of +/-2 micrometers. The web thickness variation was first measured and found to be varying some 12 micrometers above and below a median thickness of 190 micrometers. Each print that was written while a thin part of the web was on the drive roller 22 was found to be shorter than those written when the web thickness at the drive roller 22 was at its median level. The converse was true for prints made while a thick section of the web was at the drive roller. Disparities of image lengths of approximately 40 micrometers were observed; for example, a web 14 that had a thickness variation of 6 micrometers produced image frame length variations of about 30 micrometers over 177,000 micrometers of image length. When compared to an average image line width of 14 micrometers, such disparity is significant. The magnitude of the length difference was found to be directly proportional to the divergence of the web thickness from a median thickness. Hence, we noted that a solution to the problem lies in first recording the thickness variation of the web or the drum and then correlating these variation to a predicted web surface speed variation. It should be noted that the web surface speed variation due to this thickness variation would be reducible if the media could be manufactured to a precisely uniform thickness. However, such a solution is quite expensive, and is thus impractical.
The residual speed variation is not found in a non-looping web system that is driven by two pinch rollers. Because the web is not a loop and the pinch rollers deform when a thick or thin portion of the web passes therebetween, the web surface speed is the same as the median web speed. However, this configuration of web drive is uncommon.
Multicolor reproduction apparatus that produce composite color images, or other apparatus that produce color separations useable for high volume reproduction work, present strict registration requirements. In an apparatus wherein either monochromatic (separation) or multichromatic (color print) reproductions are made, i.e., for composite printing, copying, or duplicating applications, there is a significant likelihood that a disparity in image frame lengths will degrade the image quality of the composite image. Hence, the inconsistency of media transport speed becomes a significant limiting factor in obtaining quality color reproductions from such an apparatus. The production of a misregistered separation set is costly in that any subsequently-generated composite image is inaccurate and the printing process must be halted while a new set of separations are made. In the color reproduction industry, such a waste of process time is significant and is to be avoided.
Moreover, in printers and scanners that use image data that is transferred to and from a digital memory, the image frame lengths in the digitized data stream are often precisely controlled. Yet, when such images are written in an apparatus that suffers from the above-described residual speed control, this frame length precision is forfeited.